They say old habits die hard. But if every time you diet the weight comes back six months later, it may not just be your routines that are sabotaging you. A new study in the Journal of Clinical Investigation (JCI) suggests that prolonged obesity may cause structural changes in the brain, altering the parts responsible for healthy weight maintenance and ultimately undermine your weight-loss plans.

The finding is consistent with a number of recent studies that suggest that each person may have a natural “set-point” weight that our bodies try to maintain. But it’s been unclear why overweight and obese people should have their “set-points” set so high. The new JCI study is among the first to suggest some physiological mechanisms that might keep heavy people too heavy, even after they’ve begun to make healthy lifestyle changes.

“To explain a biologically elevated body weight ‘set-point,’ investigators in the field have speculated about the existence of fundamental changes to brain neurocircuits that control energy balance,” the study’s senior author, Michael Schwartz, told reporters. “Our findings are the first to offer direct evidence of such a structural change, and they include evidence in humans as well as in mice and rats.”

Although Schwartz cautions that his results are not yet the final word, here’s what the new study found:

Rodents who became obese on a high-fat diet developed persistent “neuronal injuries” in the hypothalamus — a part of the brain that regulates many metabolic functions, including hunger and fatigue. As the mice and rats continued to eat poor-quality food, their brains started to show damage in the region that regulates weight control.

Schwartz and his collaborators then looked at MRI scans from 34 obese humans, and found the same kind of unusual hypothalamus injury among them raising the possibility that a common culprit may be at work in promoting obesity across speices.

The idea of a body weight “set-point” has promising implications for the treatment of obesity. Even though it’s well known that people will lose weight any time they burn more calories than they consume, our bodies have many ways of adjusting how many calories we use during normal day-to-day functions, like muscle repair. This means that even though someone can control his calorie consumption exactly, he cannot control his total energy balance — or how much he eats minus how much he burns away — with the same kind of precision.

In addition, changes in hormone levels can also promote or suppress appetite. A 2011 study in the New England Journal of Medicine found that when obese people lost weight, they typically experienced hormonal changes that led to hunger, making it that much harder to cut calories and maintain the new, lower weight.

For the obese then, it seems like a vicious cycle. Being overweight now means that if you succeed at losing weight in the future, you still face potential challenges when you try to maintain that healthy weight. It’s not clear from the study whether the change in the hypothalamus is reversible, but the alterations may explain why people who were once heavy may struggle with hunger and find that the pounds creep back on unexpectedly — and frustratingly fast. A large number of studies confirms that successful dieters often regain much of their lost weight within one year of the diet — even when people are able to stay on the diet long-term. Seeing that weight come back can in turn quash motivation for future weight maintenance, and add to the vicious cycle.

So is dieting a futile effort for those who are overweight? Not necessarily. A healthy diet and regular exercise have well-documented health benefits, whether or not a person is overweight. The latest findings just suggest that it may be harder than simply sticking to a diet to lose weight. You might be fighting not just your eating habits, but some very powerful biological signals to eat, eat, eat.

But even if you struggle with your weight, there’s no reason to lose hope just yet. Losing any weight is better than losing now eight, and physical fitness and good nutrition can keep you feeling energetic, healthier and happier even if you never shed quite as much as you intended.